The present invention relates to a ball joint, especially a steering or suspension ball joint for motor vehicles, comprising a pivot provided with a spherical head, a ball-joint case provided with a housing for receiving the said pivot head, and a bearing shell of plastic material interposed between the pivot head and the case housing.
The invention also relates to a process for manufacture of a bearing shell for such a ball joint.
Ball joints comprising a bearing shell of plastic material are known.
According to U.S. Pat. No. 5,152,628 (=German Patent A 4032541), a bearing shell in the form of a spherical zone extending on both sides of the equator is provided on one side of the equator with a plurality of meridian slits which limit plastic deformation of the bearing shell during mounting of the spherical head of the pivot in the bearing shell by expansion of the part equipped with slits. Nevertheless, these slits have an unfavorable influence on the lubrication of the pivot. In fact, these slits exhibit rough areas and excessive thicknesses derived from the method for manufacture of the bearing shell, leading to excessive pressures and in turn an increase in the friction of the ball joint.
According to European Patent A 0653573, a bearing shell is molded in the form of a hemispherical cup, which is prolonged on its orifice side by a cylindrical skirt which, during assembly of the ball joint, is deformed plastically into a spherical segment by a locking ring of the ball joint. This does not allow control of the friction, and so large variations of the friction moment occur during mounting, during the first displacements of the pivot and throughout the entire operation of the ball joint.
For all of these reasons, the known ball joints are not satisfactory and, when used as steering ball joints, for example, lead to deterioration of the return movement of the steering, of centering around the mean position, of driving comfort and even of steering response.
The object of the present invention is a ball joint which is distinguished from known ball joints both by a reduction of friction and by a reduction of the variations of the friction moment under radial and axial load, of elasticity and of wear, throughout the entire life of the ball joint.
The ball joint which is the object of the invention comprises a pivot provided with a spherical head, a ball-joint case provided with a housing to receive the said pivot head, and a bearing shell of plastic material interposed between the pivot head and the case housing. The bearing shell of the ball joint comprises a one-piece molded cup, closed at the bottom side and provided, by molding,
with a spherical inside surface extending over a solid angle corresponding to more than one hemisphere,
with a circular orifice, whose diameter is smaller than the diameter of the spherical inside surface, opposite the bottom, and
with a plurality of rupture zones of small thickness distributed over the circumference of the orifice part, while extending from the said orifice toward the equatorial plane of the spherical surface disposed parallel to the plane of the said orifice, the said rupture zones having such a conformation that each undergoes predefined rupture into two sections during a first expansion of the orifice part and that, after insertion of the pivot head into the bearing shell, elastic constriction of the orifice part is permitted together with exact repositioning of the two sections of each rupture zone around the pivot head.
The provision of rupture zones makes it possible to mold the bearing shell in the form of a cup having a continuous spherical inside surface while remedying the drawbacks resulting, in the known ball joints, from the presence of the slits needed to permit assembly of the ball joint or plastic deformations occurring during assembly.
Preferably the rupture zones have a conformation that each gives rise to a rupture along a zig-zag line, predefined by a corresponding line of weakness.
To obtain a ball joint with a predefined friction and damping effect, the bearing shell can be provided on the outside, in the equatorial zone, with a cylindrical part, which toward the bottom zone is followed by a substantially conical part. In corresponding manner, the housing of the case is provided with a cylindrical part followed by a substantially conical bottom part.
Preferably the conical part at the bottom of the housing of the case can comprise a frustoconical outside zone, with a taper angle larger than the taper angle of the conical part of the bottom zone of the bearing shell and a conical inside (central) zone with a taper angle smaller than the taper angle of the said conical outside part of the bearing shell.
Preferably the bearing shell is provided, between its cylindrical outside part and its orifice, with a frustoconical outside part followed at the level of the orifice by an outside shoulder. The ball joint additionally comprises a locking ring which is provided with a frustoconical inside surface corresponding to the said frustoconical outside part of the bearing shell, and which, after insertion of the pivot head into the bearing shell, can be interlocked behind the shoulder of the bearing shell so as to maintain the assembled pivot and bearing shell in the form of a module capable of being mounted as such in the case of the ball joint and of being fixed therein, for example by crimping. By the dimensioning of this locking ring, it is possible perfectly to control the play between the bearing shell and the pivot of the ball joint.
The bearing shell of the ball joint according to the invention can be manufactured by advantageously molding the bearing shell between a fixed mold part whose cavity corresponds at least to the bottom part of the bearing shell, at least one movable mold part whose cavity corresponds at least to the orifice part of the bearing shell, and a movable core with spherical head, whose shape corresponds substantially to the shape of the spherical head of the pivot of the ball joint. To remove the bearing shell from the mold, there are successively displaced
a) the movable mold part, to release the orifice part of the bearing shell, or in other words the part comprising the rupture zones, toward the outside, and
b) the movable core, to extract the spherical head of the bearing shell on the orifice side, while causing well-defined rupture of the said rupture zones by expansion of the orifice part of the bearing shell.
Preferably the bearing shell is blocked in the fixed part of the mold during extraction of the spherical head of the core.